Conventionally, a gas sensor has been known which is provided with a gas sensor element that generates electromotive forces of different magnitudes in accordance with difference in concentrations of specific gases, such as NOx (nitrogen oxide) and oxygen, in an exhaust gas from an automobile or the like, and measures the concentrations of the specific gases on the basis of the magnitudes of the electromotive forces. The gas sensor is mounted to an exhaust pipe or the like of an automobile and used therein. When the gas sensor element is exposed to a high-temperature exhaust gas or heated by a heater or the like, the temperature of the gas sensor element increases. Then, if moisture contained in the exhaust gas or condensed water attached to the inner surface of the exhaust pipe is attached to the gas sensor element (if the gas sensor element is wetted by water), the gas sensor element is subjected to a thermal shock, which may cause cracking and/or breaking.
For the above reasons, a technique for covering the gas sensor element with a protector to protect the gas sensor element from water, impact, and the like has been developed (refer to Patent WO 2010/015445 A1, for example). This protector has a double structure including an inner protector and an outer protector, and is mounted to a front end of a metallic shell. The inner protector has a bottomed tubular portion that houses a front side of the gas sensor element, and a flange portion extending outward in the radial direction of the tubular portion. Inner gas introduction holes are opened penetrating through the flange portion in the vertical direction (axial direction) of the flange portion. On the other hand, an outer gas introduction hole is opened through the outer protector. A target gas introduced through the outer gas introduction hole passes through the inner gas introduction holes and reaches the gas sensor element disposed in the inner protector, and thereafter is discharged from a gas discharge hole provided at a front end of the tubular portion of the inner protector.